Edge polished wafer, polishing cloth for edge polishing, and apparatus and method for edge polishing

ABSTRACT

Provided are a novel edge polished wafer in which a wafer peripheral sag is suppressed, a polishing cloth, a polishing apparatus and a polishing method for processing the wafer. The wafer is provided by controlling an over-polish width in edge polishing to 400 μm or less. Also, the polishing cloth has a multi-layer structure of at least two layers including a polishing fabric layer an Asker C hardness of which is 65 or higher and a sponge layer an Asker C hardness of which is 40 or lower, or a single layer structure of the polishing fabric layer. Further, the polishing apparatus and the polishing method are provided by edge polishing such that the wafer in rotation is put into contact with a rotary drum having the polishing cloth adhered thereon at a prescribed angle thereto while supplying polishing slurry to the contact portion of the polishing cloth.

TECHNICAL FIELD

The present invention relates to an edge polished wafer with asuppressed sag in the outer peripheral portion thereof, to a polishingcloth for edge polishing, and to an apparatus and method for edgepolishing.

BACKGROUND ART

In recent years, there has been demanded a high flatness level forwafers, for example, thin disk-shaped wafers, such as a semiconductorsilicon single crystal wafer (hereinafter also simply referred to as asilicon wafer), a glass substrate including a quartz wafer or others, aceramic substrate including alumina, aluminum nitride or the like. Amanufacturing process for the wafers, for example, a silicon wafer,comprises: a slicing step of slicing a cylindrical semiconductor ingotwith a wire saw, a circular inner diameter blade or the like, to obtainthin disk-shaped wafers; a chamfering step of chamfering a peripheraledge portion of the wafer obtained through the slicing step to preventchipping of the periphery thereof; a lapping step of lapping bothsurfaces of the chamfered wafer to adjust a thickness and flatness levelthereof; an etching step of immersing the lapped wafer into an etchingsolution to etch all the surfaces thereof for removing processing damageremaining therein; and a polishing step of mirror polishing a singlesurface or both surfaces of the etched wafer for improving a surfaceroughness level and a flatness level thereof.

Furthermore, the chamfered portion of the above chamfered wafer isusually subjected to edge polishing prior to polishing the wafer forpreventing particle generation upon contact thereof by handling or thelike in later steps.

The edge polishing has been performed using a known edge polisher 10 asshown in FIGS. 12(a) and 12(b) (see JP A 99-188590). The edge polisher10 has a rotary drum 12 and is of a structure that the drum 12 rotatesabout a rotary shaft 14 at a high speed. A multi- or single layerpolishing cloth (a polishing pad) 16A (FIG. 12(a)) or 16B (FIG. 12(b))is fixed on all the outer surface of the rotary drum 12.

18 designates a wafer rotating device, which is installedcorrespondingly to the rotary drum 12 and can freely change a tilt anglerelative to the rotary drum 12. The wafer rotating device 18 includes abase 20, and a rotary shaft 22 rotatably installed on the top surface ofthe base 20. A wafer W is held on the top end of the rotary shaft 22 andsubjected to edge polishing. 24 designates a nozzle, which suppliespolishing slurry 26 to a contact portion of the polishing cloth 16 withthe wafer W. Edge polishing is performed by polishing the chamferedportion of the wafer W with rotating both the rotary drum 12 and thewafer W, and pressing the wafer W to the polishing cloth 16 on therotary drum 12 at a predetermined tilt angle, for example, in the rangeof from 40 degrees to 55 degrees relative to a polishing surface of therotary drum.

In the above published patent application, as the polishing cloths(polishing pads) 16A and 16B, there was disclosed as prior art not onlya polishing cloth (polishing pad) 16B (FIG. 12(b)) of a single layerstructure, but also a polishing cloth (polishing pad) 16A of a two layerstructure, that is, a multi-layer structure (FIG. 12(a)) constituted ofa sheet 16 a made of at least one of synthetic resin foam, nonwovenfabric, resin-treated nonwoven fabric, synthetic leather, or compositesthereof; and an elastic sheet 16 b such as a synthetic rubber sheet or asponge sheet.

In the above edge polishing method, in order to increase a pressed-downamount into the polishing cloth (polishing pad) 16A or 16B, there aredisclosed techniques using soft polishing cloths (for example, Suba 400having a thickness of 1.27 mm and an Asker C hardness of 61 used inComparative Example of the above published patent application; Suba 400having a thickness of 1.27 mm+synthetic rubber having a thickness of 1mm and a rubber hardness of 50 used in Example 1 of the published patentapplication; and Suba 400 having a thickness of 1.27 mm+sponge sheetused in Example 2 of the published patent application).

In the case where edge polishing is performed using a polishing cloth ofthe above noted structure, it has been found according to experiments bythe present inventor that as shown in FIG. 13 a polished portion 32 isproduced extending 500 μm or more into a wafer main surface from a waferchamfered portion 30. In this specification, hereinafter, such excesspolishing is referred to as over-polish and a width of the polishedportion 32 is referred to as an over-polish width. The width of thewafer chamfered portion 30 is generally on the order of 500 μm.Therefore, the sum of the wafer chamfered portion 30 (about 500 μm) andthe over-polish width 32 (about 500 μm) is about 1000 μm, that is, about1 mm.

An edge exclusion area (E. E.) for flatness measurement on a wafersurface is currently 3 mm from the periphery of the wafer; therefore, ifan over-polish width is about 500 μm, an influence thereof on waferflatness is small. However, there have recently increased demands forthe smallest possible edge exclusion area (E. E.) for wafer flatnessmeasurement. This is for improving a yield of device chips obtainablefrom one wafer.

Generally, there has been a tendency that while a relatively highflatness level is achieved in a central portion of a wafer, a waferthickness decreases from an inner position of about 5 mm from the waferperipheral edge to the chamfered portion, which is referred to as a sagin the outer peripheral portion of the wafer. While causes of the waferperipheral sag may be considered to be a difference between polishingpressures when polishing a wafer main surface, an influence of apolishing agent and so on, especially the sag generated at the boundaryportion between the chamfered portion and the main surface is consideredto be due to an influence of edge polishing.

For example, in case of an edge exclusion area (E. E.) for waferflatness measurement of 1 mm, an over-polish width of about 500 μm wouldinfluence a wafer flatness level and generate a sag in the outerperipheral portion of the wafer.

DISCLOSURE OF THE INVENTION

The present inventor has performed various investigations to improve theabove conventional edge polishing technique, as a result of which thepresent invention has been completed with the new findings on a novelover-polish width range enabling achievement of good wafer flatness withsuppression of a wafer peripheral sag even in case of an edge exclusionarea (E. E) for wafer flatness measurement of 1 mm, and a structure of apolishing cloth preferably used for manufacturing an edge polished waferwith the above novel over-polish width.

It is an object of the present invention to provide a novel edgepolished wafer with a suppressed sag in the outer peripheral portionthereof, a polishing cloth for edge polishing which is used formanufacturing the novel edge polished wafer with good productivity andhigh efficiency, and an apparatus and method for edge polishing with thepolishing cloth preferably used for edge polishing the above novel edgepolished wafer.

In order to solve the above problem, in an edge polished wafer accordingto the present invention, an over-polish width generated by edgepolishing is controlled to 400 μm or less.

In a wafer whose chamfered portion is mirror polished, by controllingthe over-polish width to the range of the order of 0 to 400 μm, therecan be attained a wafer with a high flatness level up close to theperipheral edge thereof through further polishing of the main surfacethereof. While an over-polish width is preferably in a state of theperfect absence (zero), in order to perfectly turn the chamfered portioninto a mirror-polished state taking into account variations inprocessing conditions, it is enough to manufacture an edge polishedwafer with an over-polish width of the order of 50 μm. Moreover, inorder to perfectly eliminate an influence of edge polishing under verystrict conditions of an edge exclusion area (E. E) for wafer flatnessmeasurement of 1 mm, an over-polish width is preferably in the range offrom 50 μm to 200 μm.

Furthermore, while various methods are conceivable for manufacturing awafer of the present invention, an especially preferable manufacturingapparatus and method are as follows: The wafer of the present inventionmay be manufactured using an apparatus comprising a rotary drum with apolishing cloth adhered on an outer surface thereof and a wafer rotatingdevice holding and rotating a wafer, wherein the wafer is edge polishedsuch that the wafer in rotation is put into contact with the polishingcloth at a prescribed angle thereto while supplying polishing slurry tothe contact portion of the polishing cloth, and the contact portion (apolishing fabric layer) of the polishing cloth with the wafer has anAsker C hardness of 65 or higher. With contrivance on the polishingcloth in use, an over-polish width is reduced and thereby a wafer havingan over-polish width of 400 μm or less can be produced stably.

Especially, a polishing cloth for edge polishing according to thepresent invention comprises at least two layers including a polishingfabric layer and a sponge layer having a hardness lower than thepolishing fabric layer being laminated, wherein an Asker C hardness ofthe polishing fabric layer is 65 or higher and an Asker C hardness ofthe sponge layer is 40 or lower. It is especially preferable that thepolishing fabric layer has a thickness of 1.3 mm or less and the spongelayer has a thickness of 1.0 mm or more. In addition, another polishingcloth according to the present invention may be of a single layerstructure including only a polishing fabric layer having an Asker Chardness of 65 or higher, in which case a thickness of the polishingfabric layer is preferably 1.3 mm or less.

A wafer with an over-polish width of 400 μm or less can be obtained bythe use of a polishing cloth having an Asker C hardness of 65 or higher.While the above wafer can be obtained using both polishing cloths of asingle layer structure and a multilayer structure provided that ahardness of a contact portion of a polishing fabric layer with a waferis the above value or higher, in case of the single layer structure thepolishing fabric layer is hard; a contact area between the wafer and thepolishing cloth in the peripheral direction is small. Consequently, itis necessary to rotate the wafer slowly in order to mirror-polish allthe chamfered portion along the peripheral portion of the wafer, leadingto a longer processing time and to poorer productivity. For this reason,an Asker C hardness of a polishing cloth of a single layer structure ispreferably 78 or less.

In case of a polishing cloth structure of at least two layer including apolishing fabric layer and a sponge layer being laminated, wherein anAsker C hardness of the polishing fabric layer is 65 or higher and anAsker C hardness of the sponge layer is 40 or lower, a contact areatoward the center of the wafer can be limited smaller, that is anover-polish width is controlled to 400 μm or less, while the contactarea between the wafer and the polishing cloth in the peripheraldirection can be large, thereby preferably, enabling reduction in timefor mirror polishing all the chamfered portion of the wafer. Especially,by adjusting hardness and thickness of the sponge layer, an applicableupper limit of the polishing fabric layer is raised, so the polishingfabric layer with an Asker C hardness of 81 or higher, for example, canbe used with almost no specific limitation provided that the polishingcloth has a hardness with which a mirror-finished surface is obtainablefree of scratches and the like; processing can be performed with anextremely reduced over-polish width and good productivity. Note thatwhile there is no specific limitation on an upper limit of the hardnessof the polishing fabric layer, it is practically sufficient if the upperlimit is an Asker C hardness of about 90. Moreover, while there is nospecific limitation on a lower limit of the hardness of the sponge layeras well, it is practically sufficient if the lower limit is an Asker Chardness of about 10.

Thus, if the polishing cloth of the two layer structure is used tomirror polish the chamfered portion of the wafer, there can be obtaineda longer contact length between the wafer and the polishing cloth in thewafer peripheral direction with a suppressed over-polish width, therebyimproving polishing efficiency and increasing productivity. In thiscase, with an excessively thick polishing fabric layer, an effect from asponge layer decreases; a thickness of the polishing fabric layer ispreferably 1.3 mm or less. With the thickness of 0.5 mm or less, alifetime of the polishing cloth is shorter to thereby increase thefrequency of exchanging the polishing cloths, so the thickness ispreferably on the order of 1.3 mm to 0.7 mm. A thickness of the spongelayer is preferably 1 mm or more. If the thickness is less than 1 mm, itreduces an effect of making larger of the contact area between the waferand the polishing cloth along the periphery thereof. Contrary to this,if the thickness is excessively large, it is difficult to adhere thepolishing cloth around the rotary drum; therefore the thickness ispreferably on the order of 1 mm to 2 mm.

As described above, the edge polished wafer can be preferablymanufactured using the apparatus with the polishing cloth having anAsker C hardness of 65 or higher under processing conditions (an edgepolishing method) that a polishing load is 2 kgf or more and a tiltangle of the wafer against the polishing cloth is in the range of from40 degrees to 55 degrees. Note that the tilt angle of the wafer againstthe polishing cloth means an angle formed between the normal of thepolishing cloth and the wafer and if the tilt angle is less than 40degrees, non-processing easily occurs in a boundary portion between thechamfered portion and a main surface of the wafer; while more than 55degrees, an over-polish width is larger and non-processing furthereasily occurs in the outermost peripheral portion of the wafer.

In order to control an over-polish width to 400 μm or less using aconventional relatively soft polishing cloth (an Asker C hardness ofabout 60), a method is conceivable that the lowest possible polishingload is applied. In this condition, however, there arise inconveniencesthat a non-polished portion is left and that it takes an extremely longtime for edge polishing. In order to perform efficient edge polishingwith the above apparatus in view of productivity, it is preferable thata polishing load is 2 kgf or more, and a tilt angle of the wafer againstthe polishing cloth is in the range of from 40 degrees to 50 degrees.Furthermore, the above efficient edge polishing may be achieved easilyby adopting the polishing cloth having an Asker C hardness of 65 degreesor more.

While no specific limitation is imposed on an upper limit of a polishingload, it is only required that a sinking amount of a wafer into apolishing cloth is confirmed depending on hardness of the polishingcloth and thereafter, the polishing load is properly determined so as toadjust an over-polish width to 400 μm or less. Practically, the upperlimit of the polishing load may be on the order of 5 kgf.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a descriptive illustration showing an example of a polishingapparatus for edge polishing according to the present invention, whereina part (a) shows the apparatus with a polishing cloth of a multilayerstructure and a part (b) shows the apparatus with a polishing cloth of asingle layer structure;

FIG. 2 is a descriptive illustration of a main part of an edge polishedwafer of the present invention;

FIG. 3 is a perspective illustration showing a wafer in an exaggeratedform;

FIG. 4 is a site map of a specimen wafer edge polished in Example 3;

FIG. 5 is a cross sectional profile of the specimen wafer edge polishedin Example 3;

FIG. 6 is a site map of a specimen wafer edge polished in ComparativeExample 2;

FIG. 7 is a cross section of the specimen wafer edge polished inComparative Example 2;

FIG. 8 is a graph showing results of values of site flatness measured atE.E. of 1 mm in Examples 1 to 4 and Comparative Examples 1 to 3;

FIG. 9 is a graph showing evaluation results of over-polish widths inExamples 5 to 19 and Comparative Examples 4 to 6;

FIG. 10 is a graph showing evaluation results of contact lengths ofsloping sections in Examples 5 to 19 and Comparative Examples 4 to 6;

FIG. 11 is a graph showing evaluation results of contact lengths of edgesections in Examples 5 to 19 and Comparative Examples 4 to 6;

FIG. 12 is a descriptive illustration showing an example of a prior artedge polishing apparatus, wherein a part (a) shows the apparatus with apolishing cloth of a multilayer structure and a part (b) shows theapparatus with a polishing cloth of a single layer structure; and

FIG. 13 is a descriptive illustration of a main part of a prior art edgepolished wafer.

BEST MODE FOR CARRYING OUT THE INVENTION

Description will be given of one embodiment of the present inventionbelow based on the accompanying drawings. It is needless to say thatexamples shown in the figures are presented by way of illustration andvarious changes and modifications of the examples may be made withoutdeparting from the technical concept of the present invention.

FIG. 1 shows an edge polishing apparatus 10 a according to the presentinvention, which is similar to the prior art apparatus 10 shown in FIG.12 in terms of basic construction, but is different therefrom in thatthere are used polishing cloths (polishing pads) 17A and 17B differentin structure, especially hardness, instead of the prior art polishingcloths (polishing pads) 16A and 16B. Therefore, there is not repeatedthe second description on the points other than the polishing cloths. InFIG. 1, the same or similar reference symbols are used to designate thesame or similar members.

In JP A 99-188590 described above as well, there are shown an example ofa polishing cloth (polishing pad) of a single layer structure (FIG.12(b)) and an example of that of a multi-layer structure constituted oftwo sheets (FIG. 12(a)). As described above, however, in the publishedpatent application there is disclosed neither necessity for controllingan over-polish width of a chamfer in processing a main surface of awafer to a high flatness level, nor teachings of a preferable hardnessof the polishing cloth 16B of a single layer structure for reaching thesolution of how to control the over-polish width, of a preferable rangein hardness of the sheets 16 a and 16 b constituting the polishing pad16A of a multilayer structure, and of the distribution ratio in hardnessbetween the sheets 16 a and 16 b.

That is, the polishing cloth 17A for edge polishing of the presentinvention is clearly differentiated from the polishing pad 16A of amultilayer structure described in the published patent application, inthat the polishing cloth 17A is, as shown in FIG. 1(a), of a multilayerstructure constituted of at least two layers, an outer polishing fabriclayer 17 a and an inner sponge layer 17 b, and by specifying hardness ofeach of the layers in the respective prescribed ranges, good edgepolishing (with an over-polish width of 400 μm or less) is enabled.

It is necessary that the polishing fabric layer 17 a is made ofnon-woven fabric, resin-treated non-woven fabric, synthetic resin foamor synthetic leather, or a composite thereof and hardness thereof is anAsker C hardness of 65 or higher and preferably 68 or higher; to bedetailed, preferably used are Suba 400 H (an Asker C hardness of 68),Suba 600 (an Asker C hardness of 78), Suba 800 (an Asker C hardness of81) made by Rodel Nitta Company and others.

Herein, an Asker C hardness indicating hardness of a polishing cloth anda sponge member is a value measured with an Asker C type rubber hardnessmeter, one kind of a spring hardness tester. This is a value inaccordance with SRIS 0101, which is a standard of the Society of RubberIndustry, Japan.

Materials used in the sponge layer are not limited in a specific way butshould have flexibility to be attachable to the rotary drum and hardnessof 40 or less in terms of an Asker C hardness. For example, sponge-likesilicone referred to as a silicone rubber sponge or simply, a siliconesponge, or the like material may be preferably used. Furthermore, thesponge layer is not limited to such sponge foam materials, and besidesthe foam materials there may be used elastically deformable materialswhose hardness falls in a desired range. Note that while the lower limitof hardness of the sponge layer is not specified either, it should bepractically on the order of 10 in terms of an Asker C hardness.

The sponge layer 17 b is set lower than the polishing fabric layer 17 ain terms of hardness and required to be of an Asker C hardness of 40 orlower. The polishing fabric layer 17 a is preferably 1.3 mm or less inthickness and the sponge layer 17 b is preferably 1.0 mm or more inthickness.

Moreover, the polishing cloth 17B for edge polishing is, as shown inFIG. 1(b), of a single layer structure constituted of a polishing fabriclayer 17 alone, but is distinctly differentiated from the polishing pad16B of a single layer structure described in the above published patentapplication in that by specifying hardness of the polishing fabric layerin a prescribed range, good edge polishing treatment (with anover-polish width of 400 μm or less) is enabled. As materials of apolishing fabric layer of the polishing cloth 17B for edge polishing,those similar to the above polishing fabric layer 17 a may be applied.

In an edge polished wafer Wa according to the present invention, anover-polish width 32 caused by edge polishing is controlled to 400 μm orless as shown in FIG. 2. With such a structure, even when an edgeexclusion area (E. E) for measurement on a wafer surface is 1 mm, thesum of the chamfer (500 μm) and the over-polish width (400 μm or less)is 900 μm or less; therefore, no peripheral sag is generated so thatflatness of the wafer is not affected thereby.

In order to manufacture an edge polished wafer Wa according to thepresent invention by the use of the polishing apparatus 10 a with thepolishing cloth 17A or 17B for edge polishing according to the presentinvention, edge polishing is performed in conditions that a polishingload is 2 kgf or more and a tilt angle of the wafer to the polishingcloth 17 is in the range of from 40 degrees to 55 degrees. Note that thetilt angle of the wafer against the polishing cloth 17 means an angleformed between the normal of the polishing cloth surface and the wafer.

By edge polishing under a pressure of 2 kgf or more in terms of apolishing load, a stock removal can be increased, with the result thatedge polishing can be stably performed even in a short time withoutleaving any unpolished portion. While no specific limitation is imposedon an upper limit of the polishing load, it is sufficient practically ifa value of the order of 5 kgf is set to the upper limit. Furthermore, bysetting a tilt angle in the range of from 40 degrees to 55 degrees, asloping section and a leading edge section of a chamfer aresimultaneously polished to be effectively edge polished so that theworking time may be reduced and the productivity may be increased. Ifthe tilt angle is less than 40 degrees, non-processing is easy to occurin a boundary portion between the chamfer and a main surface, while onthe other hand, if exceeding 55 degrees, an over-polish width increasesand further non-processing is easy to occur in the outermost peripheralsection of the wafer.

FIG. 3 is a perspective illustration showing a wafer W in an exaggeratedform, wherein a reference numeral 34 indicates a portion in the vicinityof the main surface of the wafer W referred to as a sloping section anda reference numeral 36 indicates a portion in the vicinity of theoutermost periphery thereof referred to as a leading edge section. Areference numeral 34 a indicates a contact length between the slopingsection 34 and the polishing cloth 17 and a reference numeral 36 aindicates a contact length between the leading edge section 36 and thepolishing cloth 17. The longer the contact lengths 34 a and 36 a are,the shorter the polishing time becomes; it is expected to improve apolishing efficiency to that extent.

While description will be given of the present invention in a moredetailed manner taking up examples, it is needless to say that theexamples are presented by way of illustration and should not beconstrued by way of limitation.

EXAMPLES 1 TO 4 AND COMPARATIVE EXAMPLES 1 TO 3

As specimen wafers, there were used wafers obtained by chamferingperipheral sections of wafers with a diameter of 200 mm and a crystalaxis orientation of <100>, followed by etching. A chamfer of eachspecimen wafer is subjected to edge polishing using one of 7 kinds ofpolishing cloths shown in Table 1 in an edge polishing apparatus asshown in FIG. 1, respectively.

TABLE 1 Processing conditions Polishing load  2.5 kgf Drum rotation 800rpm speed Tilt angle of 45° a stage Polishing time 150 sec. (timerequired for one rotation of a wafer) Polishing cloth Single ComparativeSuba-Lite layer cloth Example 1 Comparative Suba 400 Example 2 Example 4Suba 600 Multilayer Example 1 Suba 400H + Sponge member cloth Example 2Suba 600 + Sponge member Example 3 Suba 800 + Sponge member ComparativeSuba 400 + Sponge member Example 3

Hardness and thickness of polishing cloths and sponge members (siliconesponge) shown in Table 1 are as follows:

-   Suba-Lite (made by Rodel Nitta Company): an Asker C hardness of 53    and a thickness of 1.27 mm-   Suba 400 (made by Rodel Nitta Company): an Asker C hardness of 61    and a thickness of 1.27 mm-   Suba 400 H (made by Rodel Nitta Company): an Asker C hardness of 68    and a thickness of 1.27 mm-   Suba 600 (made by Rodel Nitta Company): an Asker C hardness of 78    and a thickness of 1.27 mm-   Suba 800 (made by Rodel Nitta Company): an Asker C hardness of 81    and a thickness of 1.27 mm-   Sponge member: an Asker C hardness of 35 and a thickness of 1.0 mm

In Table 2 there are shown over-polish widths of the specimen waferssubjected to edge polishing with polishing cloths. The over-polish widthwas obtained by performing observation on a magnified image of anoutermost peripheral section (a chamfer) of each of the specimen waferswith a video microscope to evaluate a width (length) of a mirrorpolished portion on a main surface thereof, using a boundary between thechamfer and the outermost periphery of the main surface as reference.

TABLE 2 Kinds of polishing cloths Over-polish widths ComparativeSuba-Lite 600 μm ± 50 μm Example 1 Comparative Suba 400 500 μm ± 50 μmExample 2 Comparative Suba 400 + Sponge member 500 μm ± 50 μm Example 3Example 1 Suba 400H + Sponge member 400 μm ± 20 μm Example 2 Suba 600 +Sponge member 200 μm ± 20 μm Example 3 Suba 800 + Sponge member 100 μm ±20 μm Example 4 Suba 600 150 μm ± 20 μm

The results of edge polishing show the fact that even if the samecondition is applied when hardness of the polishing cloth is low, theover polish width is varied easily. In case of an Asker C hardness of 65or lower, the variation of the over-polish width was on the order of ±50μm or more. On the other hand, in case of an Asker C hardness of 65 orhigher, it was possible to restrain the variation of the over-polishwidth to the level of the order of ±20 μm.

Then, the edge polished specimen wafers were each subjected to usualpolishing on a main surface thereof using a single wafer polishingmachine with a polishing cloth of a non-woven fabric type and apolishing agent containing colloidal silica, a stock removal thereofbeing 10 μm. A cross section and site flatness (SFQR: a cell size of25×25 mm at an E.E. of 1 mm) of the wafer were measured with an opticalnon-contact flatness measuring device.

The term SFQR (Site Front least-sQuare Range) is a value indicating thebiggest range of the unevenness against an average surface of a frontside reference that was calculated at each site (cell) with reference toflatness.

In FIGS. 4 and 5 there are shown a site map and a cross section in termsof flatness in Example 3 (Suba 800+Sponge member), respectively. As isapparent from FIG. 4, bad sites in Example 3 were 1/52=2%, a good siteyield being 98%. In Example 3, it was confirmed, as shown well in FIG.5, that no peripheral sag was perfectly generated. The good site hereinmeans a site where SFQR is 0.18 μm or less.

Furthermore, in FIGS. 6 and 7 there are shown a site map and a crosssection in terms of flatness in Comparative Example 2 (Suba 400). As isapparent from FIG. 6, bad sites in Comparative Example 2 were 21/52=40%,a good site yield being 60%. In Comparative Example 2, it was confirmed,as shown well in FIG. 7, that a peripheral sag was generated.

In FIG. 8 there are shown results of site flatness in Examples 1 to 4and Comparative Examples 1 to 3 measured at an E.E. of 1 mm. In FIG. 8,an over-polish width is assigned to the abscissa and a good site yieldrelative to a standard of flatness SFQR≦0.18 μm is assigned to theordinate. It was confirmed that at an E.E. of 1 mm with a standard offlatness SFQR≦0.18 μm, a good site yield of site flatness was about 60%at an over-polish width of 500 μm or more, for example, a good siteyield of site flatness was 62% at an over-polish width of 600 μm and agood site yield of site flatness was about 60% at an over-polish widthof 500 μm, while with a controlled over-polish width of 400 μm or less,a good site yield was improved to 90% or more.

That is, SFQR values are almost all 0.18 μm or less across a surface ofa wafer; therefore, it is essential to improve a good site yield in awafer peripheral section for increasing a good site yield, which can beachieved by controlling an over polish width. It was confirmed that bychanging kinds of polishing cloths used in edge polishing, anover-polish width was able to be controlled to 400 μm or less, which isa value greatly lowered from 500 μm achieved by the use of the polishingcloth Suba 400 (Comparative Example 2) that is mainly used in thecurrent practice.

Moreover, in measurement at an E.E. of 1 mm, a peripheral sag wasobserved in the wafer peripheral section in case of an over-polish widthof 500 μm. Even in measurement at an E. E. of 2 mm, a good site yieldsometimes decreased when an over-polish width was on the order of from500 to 600 μm. This is considered because a polishing agent easilyintrudes into an over polished portion which is excessively polished,with the result that the main surface of the wafer is affected up to aportion in the vicinity of 2 mm from the periphery thereof, althoughthis is dependent on an operating condition for polishing the mainsurface. In case of an over-polish width of 150 μm, however, the abovenoted trend was not observed.

EXAMPLES 5 TO 19 AND COMPARATIVE EXAMPLES 4 TO 6

As specimen wafers, there were used wafers obtained by chamferingperipheral sections of wafers with a diameter of 200 mm and with acrystal axis orientation of <100>, followed by etching. Processingconditions for edge polishing were as follows: six kinds of polishingcloths shown in Table 3 were used, three levels of polishing loads wereadopted, 2 kgf (Examples 5, 8, 11, 14 and 17, and Comparative Example4), 2.5 kgf (Examples 6, 9, 12, 15 and 18, and Comparative Example 5),and 3 kgf (Examples 7, 10, 13, 16 and 19, and Comparative Example 6),and edge polishing was performed with a edge polishing apparatus asshown in FIG. 1.

TABLE 3 Processing conditions Polishing loads 2 kgf, 2.5 kgf, 3 kgf Drumrotation 800 rpm speed Tilt angle of 45° a stage Polishing time A waferwas polished for 10 sec. at a fixed position without rotation Polishingcloth Single layer Comparative Suba 400 (1.27 mm) cloth Examples 4 to 6Examples Suba 600 (1.27 mm) 5 to 7 Multilayer Examples Suba 600 (0.7mm) + cloth 8 to 10 Sponge member (2 mm) Examples Suba 600 (1.27 mm) +11 to 13 Sponge member (2 mm) Examples Suba 600 (0.7 mm) + 14 to 16Sponge member (1 mm) Examples Suba 800 (0.7 mm) + 17 to 19 Sponge member(2 mm)

Values of hardness of polishing cloths and sponge members (siliconesponge) used in Table 3 were the same as those described above.Measurement was performed on an over-polish width of each of the edgepolished specimen wafers, and contact lengths of the sloping section andthe edge section shown in FIG. 3, the results of which are shown inFIGS. 9 to 11, respectively.

As is apparent from FIG. 9, in edge polishing with a soft polishingcloth (Suba 400) of a single layer structure, over-polish widths ofspecimen wafers were 500 μm or more for any of polishing loads, whilewhen a polishing cloth falls within a range of hardness conditions for apolishing cloth of the present invention, an over-polish width of eachspecimen wafer was able to be controlled to 400 μm or less with any ofpolishing cloths of a multilayer structure (Examples 8 to 19) and asingle layer structure (Examples 5 to 7). Moreover, it was confirmedthat even when the polishing load was changed and when thickness of eachlayer of the polishing cloth was changed, using any of the polishingcloths according to the present invention the over-polish width can becontrolled to 400 μm or less.

Furthermore, as shown in FIGS. 10 and 11, in the polishing cloths of amultilayer structure according to the present invention (Examples 8 to19), the contact lengths of the sloping section and the edge section areboth longer than those in the polishing cloths of a single structure(Examples 5 to 7 and Comparative Examples 4 to 6); when a rotationalfrequency of the polishing cloth (the rotary drum) and a rotation speedof the wafer are both constant, the polishing rate are increased.Therefore, it can be seen that a time required for edge polishing isreduced to that extent so that the polishing efficiency isadvantageously improved.

On the other hand, in the polishing cloths of a single structureaccording to the present invention (Examples 5 to 7), the contactlengths of the sloping section and the edge section are both shorterthan those in the polishing cloths of a multilayer structure (Examples 8to 19); a time required for edge polishing is disadvantageously longerto that extent, whereas there is no change in that an over-polish widthis controlled to 400 μm or less.

Note that the present invention is not limited to the above embodiment.While in the above Examples, description is given of the presentinvention taking up silicon wafers as examples, the present inventioncan be applied in a similar way to any other wafers such as a quartzwafer and a ceramic substrate wherein high flatness is required, theperipheral section is chamfered, and further edge polishing is necessaryfor preventing particle generation from the chamfer (for improvingsurface roughness of the chamfer).

CAPABILITY OF EXPLOITATION IN INDUSTRY:

As described above, an edge polished wafer of the present invention iscapable of suppression of a wafer peripheral sag and achievement of goodflatness. By the use of a polishing cloth for edge polishing of thepresent invention, there can be efficiently manufactured an edgepolished wafer having an over-polish width controlled to 400 μm or less.Furthermore, with an apparatus for edge polishing provided with apolishing cloth for edge polishing of the present invention and a methodfor edge polishing using the apparatus, an edge polished wafer of thepresent invention can be efficiently manufactured.

1. A polishing cloth for edge polishing comprising a multi-layerstructure of at least two layers including a polishing fabric layer anda sponge layer having a hardness lower than the polishing fabric layerbeing laminated, wherein an Asker C hardness of the polishing fabriclayer is 65 or higher and an Asker C hardness of the sponge layer is 40or lower.
 2. The polishing cloth for edge polishing according to claim1, wherein the polishing fabric layer has a thickness of 1.3 mm or lessand the sponge layer has a thickness of 1.0 mm or more.
 3. An apparatusfor edge polishing comprising a rotary drum with a polishing clothadhered on an outer surface thereof and a wafer rotating device holdingand rotating a wafer, wherein the wafer is edge polished such that thewafer in rotation is put into contact with the polishing cloth at aprescribed angle thereto while supplying polishing slurry to the contactportion of the polishing cloth, the polishing cloth in use being thepolishing cloth for edge polishing according to claim
 1. 4. An apparatusfor edge polishing comprising a rotary drum with a polishing clothadhered on an outer surface thereof and a wafer rotating device holdingand rotating a wafer, wherein the wafer is edge polished such that thewafer in rotation is put into contact with the polishing cloth at aprescribed angle thereto while supplying polishing slurry to the contactportion of the polishing cloth, the polishing cloth in use being thepolishing cloth for edge polishing according to claim
 2. 5. An apparatusfor edge polishing comprising a rotary drum with a polishing clothadhered on an outer surface thereof and a wafer rotating device holdingand rotating a wafer, wherein the wafer is edge polished such that thewafer in rotation is put into contact with the polishing cloth at aprescribed angle thereto while supplying polishing slurry to the contactportion of the polishing cloth, the polishing cloth being of a singlelayer structure including only a polishing fabric layer having an AskerC hardness of 65 or higher.
 6. An apparatus for edge polishing accordingto claim 5, wherein a thickness of the polishing fabric layer is 1.3 mmor less.
 7. A method for edge polishing a wafer having a chamferedportion, comprising the steps of: edge polishing the chamfered portionwhile controlling an over-polish width to 400 μm or less wherein edgepolishing is achieved by using one of a polishing cloth comprising amulti-layer structure of at least two layers including a polishingfabric layer and a sponge layer having a hardness lower than thepolishing fabric layer being laminated with an Asker C hardness of thepolishing fabric layer of 65 or higher and an Asker C hardness of thesponge layer of 40 or lower and a polishing cloth comprising a singlelayer structure including only a polishing fabric layer having an AskerC hardness of 65 or higher and wherein a chamfered portion of a wafer isedge polished while controlling an over-polish width to 400 μm or less.8. A method for edge polishing according to claim 7, wherein the chamferportion is edge polished such that a polishing load is 2 kgf or more, atilt angle of the wafer against the polishing cloth is in the range offrom 40 degrees to 55 degrees using an apparatus according to claim 3.9. A method for edge polishing according to claim 7, wherein the chamferportion is edge polished such that a polishing load is 2 kgf or more, atilt angle of the wafer against the polishing cloth is in the range offrom 40 degrees to 55 degrees using an apparatus according to claim 4.10. A method for edge polishing according to claim 7, wherein thechamfer portion is edge polished such that a polishing load is 2 kgf ormore, a tilt angle of the wafer against the polishing cloth is in therange of from 40 degrees to 55 degrees using an apparatus according toclaim
 5. 11. A method for edge polishing according to claim 7, whereinthe chamfer portion is edge polished such that a polishing load is 2 kgfor more, a tilt angle of the wafer against the polishing cloth is in therange of from 40 degrees to 55 degrees using an apparatus according toclaim 6.